[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <91bb0e78-7ca9-63fa-a3c4-d55294b33f27@amd.com>
Date: Wed, 18 Jan 2023 11:17:22 +0530
From: Raghavendra K T <raghavendra.kt@....com>
To: Mel Gorman <mgorman@...e.de>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>,
Vlastimil Babka <vbabka@...e.cz>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Peter Xu <peterx@...hat.com>,
David Hildenbrand <david@...hat.com>,
xu xin <cgel.zte@...il.com>, Yu Zhao <yuzhao@...gle.com>,
Colin Cross <ccross@...gle.com>, Arnd Bergmann <arnd@...db.de>,
Hugh Dickins <hughd@...gle.com>,
Bharata B Rao <bharata@....com>,
Disha Talreja <dishaa.talreja@....com>
Subject: Re: [RFC PATCH V1 1/1] sched/numa: Enhance vma scanning logic
On 1/17/2023 11:15 PM, Raghavendra K T wrote:
> On 1/17/2023 8:29 PM, Mel Gorman wrote:
>> Note that the cc list is excessive for the topic.
>>
[...]
>
>>> struct kioctx_table;
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index e4a0b8bd941c..944d2e3b0b3c 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -2916,6 +2916,35 @@ static void reset_ptenuma_scan(struct
>>> task_struct *p)
>>> p->mm->numa_scan_offset = 0;
>>> }
>>> +static bool vma_is_accessed(struct vm_area_struct *vma)
>>> +{
>>> + int i;
>>> + bool more_pids_exist;
>>> + unsigned long pid, max_pids;
>>> + unsigned long current_pid = current->pid & LAST__PID_MASK;
>>> +
>>> + max_pids = sizeof(unsigned int) * BITS_PER_BYTE / LAST__PID_SHIFT;
>>> +
>>> + /* By default we assume >= max_pids exist */
>>> + more_pids_exist = true;
>>> +
>>> + if (READ_ONCE(current->mm->numa_scan_seq) < 2)
>>> + return true;
>>> +
>>> + for (i = 0; i < max_pids; i++) {
>>> + pid = (vma->accessing_pids >> i * LAST__PID_SHIFT) &
>>> + LAST__PID_MASK;
>>> + if (pid == current_pid)
>>> + return true;
>>> + if (pid == 0) {
>>> + more_pids_exist = false;
>>> + break;
>>> + }
>>> + }
>>> +
>>> + return more_pids_exist;
>>> +}
>>
>> I get the intent is to avoid PIDs scanning VMAs that it has never faulted
>> within but it seems unnecessarily complex to search on every fault to
>> track
>> just 4 pids with no recent access information. The pid modulo
>> BITS_PER_WORD
>> couls be used to set a bit on an unsigned long to track approximate
>> recent
>> acceses and skip VMAs that do not have the bit set. That would allow more
>> recent PIDs to be tracked although false positives would still exist. It
>> would be necessary to reset the mask periodically.
>
> Got the idea but I lost you on pid modulo BITS_PER_WORD, (is it
> extracting last 5 or 8 bits of PID?) OR
> Do you intend to say we can just do
>
> vma->accessing_pids | = current_pid..
>
> so that later we can just check
> if (vma->accessing_pids | current_pid) == vma->accessing_pids then it is
> a hit..
> This becomes simple and we avoid iteration, duplicate tracking etc
>
Did more brainstorming/thought on this, I see that you meant
active_bit = (current_pid % BITS_PER_LONG);
accessing_pids |= (1UL << active_bit);
In scan path:
active_bit = (current_pid % BITS_PER_LONG);
if (!(accessing_pids & (1UL << active_bit))
goto skip_scanning;
My approach above would perhaps give more false positive, this seems
better thing to..
Thanks, will come up with numbers for this patch + your vma scan delay
patch.
>>
>> Even tracking 4 pids, a reset is periodically needed. Otherwise it'll
>> be vulnerable to changes in phase behaviour causing all pids to scan all
>> VMAs again.
>>
>
> Agree. Yes this will be the key thing to do. On a related note I saw
> huge increment in numa_scan_seq because we frequently visit scanning
> after the patch
>
[...]
Powered by blists - more mailing lists